hence has a direct human influence. Models of these effects under global
warming show little agreement, so detailed scenarios are not yet possible
and this component has been ignored in impact assessments to date.
E Vertical land movement (subsidence/uplift) due to various geological processes
such as tectonics, neotectonics, glacial-isostatic adjustment (GIA), and
consolidation. While the earth may appear stable, vertical land movement
is almost universal, although the rate of change varies significantly. During
the 20th century, land movements due to GIA appear to have been
comparable to global-mean sea-level rise. (GIA is still occurring due to the
unloading of melting ice sheets from 18 000 to 6000 years ago). In addition to
natural changes, groundwater withdrawal and improved drainage has
enhanced subsidence (and peat destruction by oxidation and erosion) in
many coastal lowlands, producing several metres subsidence in susceptible
areas over the 20th century, including some major coastal cities.
Historic sea-level trends. Global sea levels are estimated to have risen 10—20 cm
during the 20th century with no evidence of acceleration. Figure 2 shows
selected relative sea-level records from Europe for this period, measured with tide
gauges. They show variable rising trends of -1.6 mm yr\, except Helsinki where
relative sea level is falling at 2.5 mm yr\ due to GIA-induced uplift, which more
than compensates for the global-mean rise. Aberdeen is also experiencing uplift,
while elsewhere relative sea-level rise is within the range of the global-mean rise.
Interestingly, Mitrovica et al. have suggested that sea-level rise in much of
Europe is being suppressed by GIA-induced uplift in response to contemporary
melting of Greenland. While this remains a preliminary result which requires
further investigation, it illustrates the large number of factors that need to be
considered when evaluating observations and future relative sea levels.
While there is no evidence of an acceleration in sea-level rise during the 20th
century, the limited sea-level measurements from the 18th and 19th century do
show an acceleration, and geological data provide further support for this
conclusion. The important implication is that coastal environments around
the world are already experiencing rates of sea-level rise which exceed those over
J. M. Gregory, J. A. Church, K. W. Dixon, G. M. Flato, D. R. Jackett, J. A. Lowe, J. M. Oberhuber,
S. P. O’Farrell and R. J. Stouffer, Comparison of results from several AOGCMs on global and
regional sea level change 1900—2100, Climate. Dynamics, in press.
K. O. Emery and D. G. Aubrey, Sea Levels, Land Levels and Tide Gauges, Springer Verlag, New
York, 1991.
B. C. Douglas, Sea-level Change in the Era of the Recording Tide Gauge, in B. C. Douglas, M. S.
Kearney and S. P. Leatherman (eds.), Sea-level Rise: History and Consequences. Academic Press,
London, 2000, pp. 37—64.
J. X. Mitrovica, M. E. Tamisiea, J. L. Davis, and G. A. Milne, Recent mass balance of polar ice
sheets inferred from patterns of global sea-level change, Nature, 2001, 409, 1026—1029.
P. L. Woodworth, High waters at Liverpool since 1768: the UK’s longest sea-level record,
Geophys. Res. Lett., 1999, 26, 1589—1592.
I. Shennan and P. L. Woodworth, A comparison of late holocene and twentieth-century sea-level
trends from the UK and North Sea region, Geophys. J. Int., 1992, 109, 96—105.
A. Long, Late holocene sea-level change, Prog. Phys. Geogr., 2000, 24, 415—423.
R. J. Nicholls
86
warming show little agreement, so detailed scenarios are not yet possible
and this component has been ignored in impact assessments to date.
E Vertical land movement (subsidence/uplift) due to various geological processes
such as tectonics, neotectonics, glacial-isostatic adjustment (GIA), and
consolidation. While the earth may appear stable, vertical land movement
is almost universal, although the rate of change varies significantly. During
the 20th century, land movements due to GIA appear to have been
comparable to global-mean sea-level rise. (GIA is still occurring due to the
unloading of melting ice sheets from 18 000 to 6000 years ago). In addition to
natural changes, groundwater withdrawal and improved drainage has
enhanced subsidence (and peat destruction by oxidation and erosion) in
many coastal lowlands, producing several metres subsidence in susceptible
areas over the 20th century, including some major coastal cities.
Historic sea-level trends. Global sea levels are estimated to have risen 10—20 cm
during the 20th century with no evidence of acceleration. Figure 2 shows
selected relative sea-level records from Europe for this period, measured with tide
gauges. They show variable rising trends of -1.6 mm yr\, except Helsinki where
relative sea level is falling at 2.5 mm yr\ due to GIA-induced uplift, which more
than compensates for the global-mean rise. Aberdeen is also experiencing uplift,
while elsewhere relative sea-level rise is within the range of the global-mean rise.
Interestingly, Mitrovica et al. have suggested that sea-level rise in much of
Europe is being suppressed by GIA-induced uplift in response to contemporary
melting of Greenland. While this remains a preliminary result which requires
further investigation, it illustrates the large number of factors that need to be
considered when evaluating observations and future relative sea levels.
While there is no evidence of an acceleration in sea-level rise during the 20th
century, the limited sea-level measurements from the 18th and 19th century do
show an acceleration, and geological data provide further support for this
conclusion. The important implication is that coastal environments around
the world are already experiencing rates of sea-level rise which exceed those over
J. M. Gregory, J. A. Church, K. W. Dixon, G. M. Flato, D. R. Jackett, J. A. Lowe, J. M. Oberhuber,
S. P. O’Farrell and R. J. Stouffer, Comparison of results from several AOGCMs on global and
regional sea level change 1900—2100, Climate. Dynamics, in press.
K. O. Emery and D. G. Aubrey, Sea Levels, Land Levels and Tide Gauges, Springer Verlag, New
York, 1991.
B. C. Douglas, Sea-level Change in the Era of the Recording Tide Gauge, in B. C. Douglas, M. S.
Kearney and S. P. Leatherman (eds.), Sea-level Rise: History and Consequences. Academic Press,
London, 2000, pp. 37—64.
J. X. Mitrovica, M. E. Tamisiea, J. L. Davis, and G. A. Milne, Recent mass balance of polar ice
sheets inferred from patterns of global sea-level change, Nature, 2001, 409, 1026—1029.
P. L. Woodworth, High waters at Liverpool since 1768: the UK’s longest sea-level record,
Geophys. Res. Lett., 1999, 26, 1589—1592.
I. Shennan and P. L. Woodworth, A comparison of late holocene and twentieth-century sea-level
trends from the UK and North Sea region, Geophys. J. Int., 1992, 109, 96—105.
A. Long, Late holocene sea-level change, Prog. Phys. Geogr., 2000, 24, 415—423.
R. J. Nicholls
86